Regeneration - Resins - Decolourization

Resins regeneration is performed normally using an alkaline salt solution at 100 g/l of NaCl. However this system is not very efficient for colourants removal from styrenic resins (Williams and Bhardwaj, 1988; Bento, 1992). In fact, decolourization capacity of these resins decreases quickly with resin life, when this kind of regeneration is used. This problem is due to colourants that stay irreversibly attached to the resin, after regeneration.

The following equation represents the decolourization and regeneration of resins:

Res ⁺ Colourant ^- + Cl ^-   <====>    Res ⁺ Cl ^- + Colourant ^-

The utilization of an alkaline salt solution helps colourants removal, although it does not solve completely this problem. Salt must be alkalinized as colourants pK are higher than 9 (Riffer, 1988). Therefore, salt solutions must have a pH higher than this value in order to change weak acids, fixed to resin matrix, into their polar anionic form. In this form, anionic compounds are more easily removed from resins.

When a salt regeneration is performed, the majority of colourants are removed at low salt concentrations, lower than 50 g/ of NaCl) (Bento, 1989).

Resins regeneration with NaCl at 100g/l

Considering this fact, resin regeneration with two salt concentrations was tested: first part with NaCl at 50 g/l and a second part with NaCl at 100 g/l (Bento, 1989). With this kind of regeneration, colourants with low anionic charge, fixed by hydrophobic inter-action, are removed at a higher quantity. Colourants with a higher anionic charge are removed in the second part, at high sat concentration. It was observed that the quantity of total colourants removed with this regeneration is greater than with normal regeneration (Bento, 1989).

Regeneration with two concentrations of NaCl

Colourants separated in these two steps, low and high salt concentration, present different degrees of ionic charges. This fact was proved by the capacity of colourants precipitation with lime (LV- “Lime Value”) (Bento, 1989). By the data in the previous Figure, it is observed that colourants removed with higher salt concentrations, present higher LV, indicating a higher anionic charge. These colourants present also a higher Hlf value Bento, 1995).

Hlf value through NaCl regeneration

Guimarães et al., 1999, observed that 80,6% of melanoidins and 62% of caramels are removed at low salt concentration, and 61,1% of HADP are removed at high salt concentrations (corresponding to 30.5% of HADP on input liquor).

In classic regeneration, with NaCl at 100 g/l, resins contact with saline solutions at various concentrations. At the beginning of regeneration, resins are immersed in water. When salt solution enter the resin columns, salt concentration increases progressively till 100 g/l of NaCl. At the end of regeneration, washing water enters the column and salt concentration concentration decreases at almost zero concentration.

At high salt concentrations colourants bond ionically to resins are removed. At these concentrations there is an increase of hydrophobic inter-action, and colourants fixed to resin matrix will not be released. In this phase ,ion exchange occurs, between anionic colourants fixed to the resin and chloride ions in solution. When resin is washed, salt concentration decreases and colourants fixed to resin matrix are released. From these colourants, those that have an anionic charge can be fixed ionically to the resin, as chloride concentration is low. This effect, named "switch effect" decrease decolourization capacity of anionic resins (Bento, 1992).

Switch effect

In order to avoid this re-fixation of colourants, released from resin matrix, an acid wash is made. Anionic colourants will not be ionized at low pH, and will be not fixed ionically to the resin. By making this acid wash it is observed an extra removal of colourants from resin, at low pH conditions. With the industrial implementation of this system, resin life increased from about 250 to 400 cycles.

Regeneration with NaCl with acid washing

In order to remove colourants fixed to resins, both ionically and by inter-action hydrophobic, both of these mechanisms must be suppressed, simultaneously. If this is not done, colourants can switch from one form to the other as it was referred. In order to make this simultaneous suppression, regeneration must be done at low chloride concentrations or with other special regenerations as sacarate regeneration.

Bento L.S.M., 1989, Sugar decolourization by ion-echange resins with
          regenerant recovery, Proc. of S.I.T.
          Conf., 176-200
Bento L.S.M., 1992, Organic and inorganic compounds influence on the sugar
colourant - ion exchange
          inter-action, Proc. of S.I.T. Conf., 201-220
Bento L.S.M., 1995, Application of UV spectrophotometry to study sugar
colourants through the refining
          process, Proc. of S.I.T. Conf., 211-230
Guimaraes C., L.S.M. Bento, M. Mota, 1996, A study of sugar colourants through
ion exchange and salt
         regeneration, Int. Sugar J., 98, 584-587
Riffer R., 1988, The nature of colorants in sugar cane and cane sugar
          manufacture, in Chemistry and  processing of sugar beet and sugar cane, Ed.
M.A. Clarke, M.A. Godshall, Pub. Elsivier,
         Amesterdam, 186-207
William J.C., C.L. Bhardwaj, 1988, The use of HPLC to investigate the
mechanism of resin decolorization,
         Proc. of S.P.R.I. Conf., 37-61

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